Energy Efficient Windows

This entry is a work in progress - more to go!

Homes are insulated to prevent heat loss. A wall insulating value of R-19 is usual while any window will be less. It makes little sense to insulate walls well only to lose excessive heat through windows. One square foot of glass can lose as much heat as ten square feet of wall.

Climate plus heat loss/gain is the basis for the sizing of the heating cooling system. Excessive heat loss/gain requires a larger heating/cooling system to compensate while maintaining a comfortable temperature inside the dwelling. This is extra expense on a heating/cooling system which could go toward better windows or into ones pocket.

Traditional single pane (single sheet of glass) windows are known to be energy inefficient. Over the years two pane and three pane windows have become popular due to their increased efficiency at restricting the transfer of heat.

- single pane windows typical U (resistance to heat flow) value of 1.1 or greater typical

- dual pane windows typical U value of 0.55 or lower typical 

- triple pane windows typical U value down to 0.20

An example of the affect of windows on the cost of cooling a 2000 ft2 home in Phoenix, AZ follows:

A. Aluminum frame with single pane clear glass - basis for the calculated savings below

B. Aluminum frame with single pane and single tint - 6% savings

C. Wood/vinyl frame with double pane clear glass - 16% savings

D. Wood/vinyl frame with double pane glass with low solar gain - low-E features - 32% savings

An example of the affect of windows on the cost of heating a 2000 ft2 fome in Boston, MA follows:

A. Aluminum frame with single pane clear glass - basis for the calculated savings below

The two examples above are from www.efficientwindows.org

The savings are significant in either case but it is confusing to determine which features one wants and at what cost.

Orientation:

In building a new house the orientation is important and can have great affect on the heating.cooling cost of the house for a lifetime. With uninsulated windows the difference in heating.cooling costs between different orientations can be great. With double and triple pane windows or with low solar gain, low-E features the difference is substantially less.

A. Heating climate - south facing windows can assist with winter heating costs.

B. Cooling climate - north facing windows are probably more suitable.

C. In any climate east and west facing windows are more difficult to shade to prevent excessive heat gain. 

D. If a window is desired in a less than desireable orientation for view or for aesthetical reasons then consider the size needed.

Gas fill: 

Multipane windows have a space between the panes that is normally filled with nitrogen. Argon or krypton are sometimes used in place of nitrogen as they have better insulating capabilities. Two layers of glass are typically used in warmer or cooler climates while the three layer version have lower heat loss characteristics and are more attractive in the coldest climates. 

A. Nitrogen

Nitrogen fill is normally utilized unless otherwise specified. 

B. Argon:

Argon has better (lower) heat transfer characteristics than nitrogen. It slowly evaporates from the cavity over time causing a loss of insulating characteristics but it is generally accepted that until the argon concentration is less than 75% the window retains it's ability to insulate. Industry standard considers the evaporation to reach the 75% concentration level will take 20 plus years to happen. Argon comes from air separation plants making it a by-product of oxygen generation. The additional cost per window as compared to using a nitrogen fill is small.

C. Krypton:

Krypton has better insulating properties than argon however it is considerably more expensive and not normally used.  

Window frames:

The frame can be a significant source of heat loss. Various types of frames are offered that may be desireable for aesthetical or fininacial reasons.  

Various materials used for window frames include: 

A. Wood  - the oldest and still a favorite - good insulating values and appearance but has maintenance requirements.

B. Steel - Strong with a long life - poor insulating values and requires maintenance

C. Aluminum - Reasonably strong with long life but poor insulating values

D. Aluminum with thermal break - All the advantages of aluminum but lower heat loss - the frame is split into two sections with a PVC type of material separating the two slowing down the heat transfer from one side to the other.. 

E. PVC/Vinyl - Long life and excellent insulating values though not strong

F. Fiberglass or composite - Newer style with long life and excellent insulating values 

Solar Heat Gain Cofficient (SHGC):

SHGC is the fraction of solar radiation passed through the window. The value can range between 0 and 1. A high SHGC means more solar gain in the home which is good in winter and bad in summer. The summer problem can sometimes be overcome by use of awnings or similar. 

Visible Light Transmission (VT): 

Visible light transmission through glazing can vary between 0 and 1 (with 1 being no loss  of transmission) though more typical figures are between 0.3 to 0.8. Use of a second pane or any coating will lower the VT value.

Low-E Coatings (Low-E):

Low-E stands for low Emittance and refers to a very thin metallic or metallic oxide layer deposited on one of the glass surfaces. Heat gain/loss through a window is due to radiation from either outside in or inside out. The coating blocks a significant portion of the radiation therefore reducing heat flow through the window. The coating is normally invisible to the casual observer. 

The Low-E coatings are sometimes used 

Air Leakage (AL):

In the event the casing of a window or any sliding components are not well sealed air will leak in or out causing loss of energy. Air leakage  is a general problem with older houses. Sealants can be used around the frames and weather strips around moving sections to reduce air movement. Sliding glass doors are the most difficult to seal.

Hydrophilic Coatings: 

Resources:

1. Energy-Efficient Window Technologies - North Carolina Cooperative Extension Service

2. www.energyefficientwindows.org

3. www.efficientwindows.org - this site has a tool to compare various configurations for a 2000 ft2 house

Additional terms:

1. R value describes a materials resistance to the flow of heat.

2. U value is the inverse of the R value and is normally used in describing window (fenestration) products. A U value of 0.25 is equal to a R value of 4   (1/0.25=4).